The majority of the world's energy, particularly for transportation, is supplied by petroleum derived fuels, which have a finite supply. Alternative sources which are renewable are needed, such as from biologically produced oils.
Triacylglycerol Biosynthesis
Triaclyglycerols (TAG) constitute the major form of lipids in seeds and consist of three acyl chains esterified to a glycerol backbone. The fatty acids are synthesized in the plastid as acyl-acyl carrier protein (ACP) intermediates where they can undergo a first desaturation catalyzed. This reaction is catalyzed by the stearoyl-ACP desaturase and yields oleic acid (C18:1Δ9). Subsequently, the acyl chains are transported to the cytosol and endoplasmic reticulum (ER) as acyl-Coenzyme (CoA) esters. Prior to entering the major TAG biosynthesis pathway, also known as the Kennedy or glycerol-3-phosphate (G3P) pathway, the acyl chains are typically integrated into phospholipids of the ER membrane where they can undergo further desaturation. Two key enzymes in the production of polyunsaturated fatty acids are the membrane-bound FAD2 and FAD3 desaturases which produce linoleic (C18:2Δ9,12) and α-linolenic acid (C18:3Δ9,12,15) respectively.
TAG biosynthesis via the Kennedy pathway consists of a series of subsequent acylations, each using acyl-CoA esters as the acyl-donor. The first acylation step typically occurs at the sn1-position of the G3P backbone and is catalyzed by the glycerol-3-phosphate acyltransferase (sn1-GPAT). The product, sn1-lysophosphatidic acid (sn1-LPA) serves as a substrate for the lysophosphatidic acid acyltransferase (LPAAT) which couples a second acyl chain at the sn2-position to form phosphatidic acid. PA is further dephosphorylated to diacylglycerol (DAG) by the phosphatidic acid phosphatase (PAP) thereby providing the substrate for the final acylation step. Finally, a third acyl chain is esterified to the sn3-position of DAG in a reaction catalyzed by the diacylglycerol acyltransferase (DGAT) to form TAG which accumulates in oil bodies. A second enzymatic reaction, phosphatidyl glycerol acyltransferase (PDAT), also results in the conversion of DAG to TAG. This reaction is unrelated to DGAT and uses phospholipids as the acyl-donors.
To maximise yields for the commercial production of lipids, there is a need for further means to increase the levels of lipids, particularly non-polar lipids such as DAGs and TAGs, in transgenic organisms or parts thereof such as plants, seeds, leaves, algae and fungi. Attempts at increasing neutral lipid yields in plants have mainly focused on individual critical enzymatic steps involved in fatty acid biosynthesis or TAG assembly. These strategies, however, have resulted in modest increases in seed or leaf oil content. Recent metabolic engineering work in the oleaginous yeast Yarrowia lzpolytica has demonstrated that a combined approach of increasing glycerol-3-phosphate production and preventing TAG breakdown via β-oxidation resulted in cumulative increases in the total lipid content (Dulermo et al., 2011).
Plant lipids such as seedoil triacylglycerols (TAGs) have many uses, for example, culinary uses (shortening, texture, flavor), industrial uses (in soaps, candles, perfumes, cosmetics, suitable as drying agents, insulators, lubricants) and provide nutritional value. There is also growing interest in using plant lipids for the production of biofuel.
To maximise yields for the commercial biological production of lipids, there is a need for further means to increase the levels of lipids, particularly non-polar lipids such as DAGs and TAGs, in transgenic organisms or parts thereof such as plants, seeds, leaves, algae and fungi.